Why does Dr. Santiago call Marcellus a "smart cookie" (73)?

Marcellus is given a so-called puzzle--a hinged box made of clear plastic with a latch on the lid. There is a crab inside. Terry lowers it into the tank, and he and Dr. Santiago watch to see how long it takes Marcellus to open the box and eat the crab. It takes Marcellus only seconds to open the box and consume the crab. Dr. Santiago comments that Marcellus is a "smart cookie."